This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A043305 #30 Mar 21 2023 09:22:52
%S A043305 1,3,6,7,15,21,28,31,33,42,69,84,91,93,105,127,135,141,186,217,231,
%T A043305 270,273,285,381,420,465,483,496,546,573,651,762,775,819,861,868,889,
%U A043305 924,945,987,1023,1149,1185,1302,1365,1419,1485,1488,1561,1638,1743,1890
%N A043305 Numbers k such that the numerator of the sum of the reciprocals of the divisors of k (=A017665(k)) is a power of 2.
%C A043305 After 1, a subsequence of A216782. If both x and y are terms and gcd(x, y) = 1, then x*y is also present. - _Antti Karttunen_, Mar 20 2023
%H A043305 Amiram Eldar, <a href="/A043305/b043305.txt">Table of n, a(n) for n = 1..10000</a> (terms 1..910 from Harvey P. Dale)
%t A043305 Select[Range[2000],IntegerQ[Log[2,Numerator[Total[1/Divisors[#]]]]]&] (* _Harvey P. Dale_, Nov 29 2014 *)
%o A043305 (PARI) isok(n) = (ispower(num = numerator(sigma(n)/n), , &s) && (s == 2)) || (num == 2) || (num == 1); \\ _Michel Marcus_, Nov 21 2013
%o A043305 (PARI) isA043305(n) = { n=sigma(n)/gcd(sigma(n),n); !bitand(n,n-1); }; \\ _Antti Karttunen_, Mar 20 2023
%Y A043305 Cf. A017665, A216782, A361465 (characteristic function).
%Y A043305 Subsequences: A000396, A336702, A348943 (odd terms).
%K A043305 easy,nonn
%O A043305 1,2
%A A043305 _Benoit Cloitre_, Apr 04 2002